Since the discovery of superconducting materials having critical temperatures that exceed the temperature of liquid nitrogen, there has been a concerted effort to utilize these materials for various applications, such as in wires and electronic devices. In order to be commercially viable, these applications require high temperature superconducting materials with large critical current density. Critical current density, J.sub.c, is the maximum current density a superconductor can carry at a given temperature and magnetic field. One such high temperature superconducting (HTS) material is a composite oxide of RE, Ba and Cu, (ReBCO) and in particular, REBa.sub.2 Cu.sub.3 O.sub.x (wherein RE represents at least one of the following rare earth elements: Y, La, Sm, Nd, Eu, Gd, Dy, Ho, Er, Tm, Yb, or Lu).
Current materials research aimed at fabricating high temperature superconducting ceramics in conductor configurations for bulk, practical applications, is largely focused on powder-in-tube methods. Such methods have proven quite successful for the Bi--(Pb)--Sr--Ca--Cu--O (BSCCO) family of superconductors due to their unique mica-like mechanical deformation characteristics. In high magnetic fields, however, this family of superconductors is generally limited to applications below 30.degree. K. In the ReBCO Tl--(Pb, Bi)--Sr--(Ba)--Ca--Cu--O and Hg--(Pb)--Sr--(Ba)--Ca--Cu--O families of superconductors, some of the compounds have much higher intrinsic limits and can be used at higher temperatures.
It has been demonstrated that these superconductors possess a high J.sub.c at high temperatures when fabricated as single crystals or in essentially single-crystal form as epitaxial films on single crystal substrates such as SrTiO.sub.3 and LaAlO.sub.3. An epitaxial film is one whose crystalline lattice is nearly perfectly aligned with the lattice of the substrate on which it is deposited. These superconductors have so far been intractable to conventional ceramics and materials processing techniques to form long lengths of a polycrystalline conductor with a J.sub.c comparable to epitaxial films. This is primarily because the poor electrical connections at the boundaries between crystalline grains, which is known in they art as the "weak-link" effect.
Thin-film materials having perovskite-like structures are important in superconductivity, ferroelectrics, and electro-optics. Many applications using these materials require, or would be significantly improved by single crystal, c-axis oriented perovskite-like films grown on single-crystal or highly aligned metal or metal-coated substrates. For instance, Y--Ba.sub.2 --Cu.sub.3 --O (YBCO) is an important superconducting material for the development of superconducting current leads, transmission lines, motor and magnetic windings, and other electrical conductor applications. When cooled below their transition temperature, superconducting materials have no electrical resistance and carry electrical current without energy dissipation.
One technique for fabricating a superconducting wire or tape is to deposit a YBCO film on a metallic substrate. Superconducting YBCO has been deposited on polycrystalline metals in which the YBCO is c-axis oriented, but not aligned in-plane. To carry high electrical currents, however, the YBCO films must be biaxially textured, preferably c-axis oriented, with effectively no large-angle grain boundaries, since such grain boundaries are detrimental to the current-carrying capability of the material. YBCO films deposited on polycrystalline metal substrates do not generally meet this criterion.
Many electronic, magnetic, or superconductor device applications require control of the grain boundary character of the device materials. For example, grain boundary character is very important in high temperature superconductors. It is known that the critical current density through a grain boundary may be reduced significantly for misorientation angles greater than 5.degree.-10.degree.. It is thus desirable to obtain superconducting deposits in which the number of grain boundaries with misorientation angles greater than 5.degree.-10.degree. is minimized. For conductors in which the superconducting deposit is epitaxial with an underlying metallic or oxide buffer layer or substrate, it is desirable to minimize the number of grain boundaries with misorientations greater than 5.degree.-10.degree..This is accomplished if the texture of the substrate has grain orientations which vary by no more than 5.degree.-10.degree.. Useful superconducting layers may be obtained using substrates with a larger spread in grain orientation. However, the properties of the superconductor deposit are expected to improve with a biaxially textured substrate having a narrow spread in grain orientation.
The effects of grain boundary characteristics on current transmission have been clearly demonstrated for certain materials, for example, the material known as YBCO. See Dimos, et al. (1988) Phys. Rev. Lett. 61:219; and Dimos, et al. (1990) Phys. Rev. Lett. 41:4038. For clean, stoichiometric boundaries, the grain boundary critical current (J.sub.c (gb)) appears to be determined primarily by grain boundary misorientation. The dependence of J.sub.c (gb) on misorientation angles for YBCO has been determined by Dimos et al. for grain boundary types which can be formed in epitaxial films on bicrystal substrates. These include [001] tilt, [100] tilt, and [100] twist boundaries. In each case, however, high angle boundaries were found to be weak-linked.
Recently, the Dimos work has been extended to artificially fabricated [001] tilt bicrystals in Tl.sub.2 Ba.sub.2 CaCu.sub.2 O.sub.8 (A. H. Cardona, et al., Appl. Phys. Lett., 62 (4), 411, 1993)), NdI.sub.0.85 Ce.sub.0.15 CuO.sub.4, Tl.sub.2 Ba.sub.2 Ca.sub.2 Cu.sub.3 O.sub.x (M. Kawasaki, et al., Appl. Phys. Lett., 62(4), 417 (1993)), and TlBa.sub.2 Ca.sub.2 Cu.sub.2 O.sub.x (T. Nabatame, et al., Appl. Phys. Lett. 65 (6), 776 (1994)). In each of these cases, it was found that, as in the case of YBCO, J.sub.c depends strongly on grain boundary misorientation angle. Although no measurements have been made on the material known as Bi-2223, data on current transmission across artificially fabricated grain boundaries in the material termed Bi-2212 indicate that most large angle [001] tilt (M. Kawasaki, et al., Appl. Phys. Lett., 62 (4), 417 (1993)) and twist boundaries are weak links, with the exception of some coincident site lattice (CSL) related boundaries (N. Tomita, et al., Jpn. J. Appl. Phys., 29 (1990) L30; N. Tomita, et al., Jpn. J. Appl. Phys., 31, L942 (1992); J. L. Wang, et al., Physica C, 230, 189 (1994)). It is likely that the variation in J.sub.c with [100] tilt grain boundary misorientation in materials Bi-2212 and Bi-2223 is similar to that observed in the well characterized cases of YBCO and Tl-based superconductors.
Hence, in order to fabricate high temperature superconductors with a high J.sub.c, it is necessary to have good biaxial texture. This has been shown to result in significant improvement in the superconducting properties of YBCO films (Y. Iijima, et al., Appl. Phys., 74,1905 (1993); R. P. Reade et. al., Appl. Phys. Lett., 61, 2231 (1992); X. D. Wu, et al., Appl. Phys. Lett., 65, 1961 (1994).
Methods have been developed to biaxally texture ReBCO to obtain a high J.sub.c. High J.sub.c 's have been reported in polycrystalline ReBCO in thin films deposited in special substrates on which a biaxially textured non-superconducting oxide buffer layer is first deposited using ion-beam assisted deposition (IBAD) techniques. High J.sub.c 's have also been reported in polycrystalline ReBCO melt-processed bulk material which contains primarily small angle grain boundaries.
Recent developments in biaxially textured metallic substrates such as Rolling Assisted Biaxially Textured Substrates (RABiTS), such as described in U.S. Pat. Nos. 5,739,086 and 5,741,377, which are fully incorporated by reference herein, and IBAD on Hastelloy have enabled the fabrication of high J.sub.c high temperature superconductors (HTS) including YBa.sub.2 Cu.sub.3 O.sub.x and (Tl, Bi)-1223.
In the RABiTS process, metallic tapes are obtained from deformation processes such as rolling. These tapes are then annealed at elevated temperatures to enable the development of biaxially aligned cube texture. During the annealing process, however, surface irregularities, such as thermal grooves as shown in FIG. 1, appear and are frequently retained through subsequent buffers and HTS deposition. The presence of these irregularities in the HTS film reduces the current carrying ability of the superconductor. Therefore, it is highly desirable to eliminate, or at least minimize, these irregularities so as to achieve a more reproducible and useful superconductor performance.
In addition to the problems associated with surface irregularities, another drawback that is common to biaxially textured substrates, including those formed using RABiTS and IBAD processes, is the brittle nature of the ceramic buffer layers. These buffers serve as diffusion barriers as well as transitional layers to obtain final HTS epitaxy. To date, high J.sub.c HTS films have been deposited onto these textured substrates only by vapor deposition methods.
For large scale manufacturing, it is highly desirable to deposit the HTS by alternative means such as powder deposition and solution precursor prior to high temperature HTS formation treatments in an oxidizing atmosphere. However, these large scale manufacturing methods require densification of the precursor prior to the HTS formation treatments. Due to the brittle nature of the ceramic buffers, cracks will be initiated and will propagate within the buffers. Discontinuities, such as cracks, will permit the diffusion of undesirable elements into the HTS as well as the formation of rough metallic oxides, such as shown in FIG. 2, thereby severely degrading the superconducting properties of the coated conductor.
For further background information, refer to the following publications:
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